belts

I just recently got myself an 88 Ford Ranger with a 2.9. Most everything seems to be pretty good on it except there was no belt on the AC. I went ahead and put on one to see if it works and everything there seemed ok, until I popped the hood last night and saw the belt was being shredded after...

My 2.9L with an A4LD has begun making this noise at around 4000 RPMs, but once I drop below or gain about 500 RPMs the noise goes away. What gives? Sorry for posting twice in such a short time frame, just trying to get as much information as possible.